The present invention relates to a control circuit for controlling a solenoid-operated actuator in a closed loop in response to a current that drives the actuator. The invention is particularly suitable for fluid-regulated systems in a wide range of automotive applications.
Conventional actuator control circuits comprise a switching element connected in series with the solenoid coil of the actuator and arranged to response to an electrical pulse by generating a drive current in the coil. The switching element is connected in parallel with a Zener diode and further connected in series with a resistor through which the drive current is drained. When the switching element is turned off, surge current is generated which flows through the Zener diode and the resistor. The voltage developed across the resistor is compared with a control signal to derive the switching control pulse. Shortcomings inherent in the prior art control circuit are the production of excessive heat in the Zener diode due to the heavy surge current and the difficulty in providing resistances appropriate for detection of drive and surge currents to generate feedback control signals.